In the welding process commonly referred to as gas metal arc welding an electrical arc is established between a consumable metal electrode and the workpieces which are to be joined. The arc transfers molten metal from the electrodes to form the weld bead. Oxidation of the molten metal is reduced and the arc is stabilized by directing a flow of shielding gas to the weld region. The shielding gas is typically composed of one or more inert gases containing a small admixture of oxygen or carbon dioxide which creates a mildly oxidizing atmosphere that promotes fusion of the electrode metal with that of the workpieces.
It is known practice to cyclically pulse the arc current between a minimal value needed to maintain the arc and a maximum value which may be several hundred percent larger. Pulse frequency may range from several cycles per second up to several hundred cycles depending on conditions at the particular welding operation. This results in a transfer of molten electrode metal in the form of periodic large droplets rather than as a continuous spray which is more difficult to control and which is less suited to so called out of position welding where the weld bead must be formed with a vertical or inclined orientation.
Significant limitations and problems are encountered in the conventional practice of pulsed arc gas arc welding.
It would be advantageous if the energy input needed to produce a given length of weld bead were reduced and the rate of deposition of metal increased. Heat adversely affects the metallurgical properties of metals and a reduction of energy input would reduce the extent of heat damage. Faster metal deposition would increase productivity. It would also be advantageous if the process were less sensitive to the skills of the individual welders.
Welds produced by conventional pulsed arc gas metal arc welding typically require extensive mechanical reworking to eliminate defects and to improve appearance. Heavy oxidation may be present. Spattering, sagging, surface irregularities and non-uniform bead profiles are common occurrences. Fusion or penetration is sometimes poor and may necessitate rejection of a weld. Elimination or reduction of the need for reworking and a reduction of rejection rate would clearly be advantageous.
The above discussed problems are encountered in working with weldable ferrous metals in general but are particularly acute in the welding of certain specific metals or classes of metals. Stainless steels, low alloy steels and nickel based alloys are the more common examples. In addition to the problems discussed above, losses of specific components of such alloys occur when prior pulsed arc gas metal welding equipment and techniques are used. Such welding of stainless steel, for example, tends to deplete the chromium content of the metal which effect is believed to result from selective oxidation or evaporation of that particular component. The resulting weld bead may be subject to rusting and may be deficient in other desirable properties of stainless steel.
The present invention is directed to overcoming one or more of the problems discussed above.